The wide proliferation of IR missiles both air-air and surface-to-air has led to the military development of a variety of infrared countermeasure systems. This includes such systems as cued IR flares, towed IR decoys, omnidirectional on-board jammers and lamps and laser based directable jammers.
Of these types the only effective jammers for protection of large aircraft against the large inventory of missiles is the directable laser jammer, also known as DIRCM or the Directed Infrared Countermeasure System.
DIRCM systems operate based on a cue from a missile's warning system that slews a pointing and tracking sensor to track the threat missiles and then emits laser jammer radiation onto the missile dome. These systems are co-located on the aircraft and emit modulated waveforms which deceive the missile guidance. The on-board systems are designed to operate on the centerline of the missile's axis.
Commercial aircraft and other aircraft which are not protected by active jamming systems such as the directed infrared countermeasure systems, are particularly vulnerable to shoulder-launched missiles especially when the aircraft descends below 10,000 feet, currently the effective maximum altitude for such missiles.
Military aircraft carry a wide variety of DIRCM systems the purpose of which is to detect an incoming missile usually by detecting its plume, and then gimbling the laser optics to project a modulated infrared laser beam directly on-axis so as to countermeasure the guidance system for the missile by causing it to veer off the target.
Typically, the missile is aimed directly at the targeted aircraft such that providing a modulated laser beam directly towards the missile to countermeasure the missile jams the missile's guidance system. To do this the laser beam impinges on the transparent dome protecting the missile's seeker directly along the missile's centerline at a 0° intercept angle.
It has been found when the laser beam impinges on the transparent dome of the missile at angles greater than 3°, the amount of jamming radiation reaching the missile's IR detector is significantly reduced. Thus, in the past it was thought that any effective laser jamming of the missile had to involve the head-on illumination of the missile's seeker. Off-axis illumination of the missile's seeker was found not to be particularly effective.
Note that for aircraft-carried DIRCMs, the success rate of countermeasuring infrared seeker missiles has been exceedingly high. The problem however in providing commercial aircraft with DIRCMs is both a perceptional problem from the point of view of the passengers and also a cost problem. Moreover, there is a problem of retrofitting the many existing commercial aircraft even if cost is not an issue. In order to retrofit a commercial aircraft, one has at the very least to mount a pod on the aircraft, which pod includes cutting a hole in the skin of the aircraft, thus breaching airframe integrity. Note also that the current cost of the DIRCM hardware is on the order of one million dollars, with the cost of retrofitting the aircraft being an additional one million dollars.
If cost where not enough of a deterrent for commercial aviation, the provision of a pod on a commercial aircraft is clearly visible by passengers and is frightening to them. This impediment in addition to having implications for drag, fuel efficiency and logistics presents a challenge. Thus having the infrared countermeasure pod visible creates passenger anxiety. Also having a large crew required for maintenance, testing and boresighting at each turn around for the plane results in a small army of people descending on the plane to ready the DIRCM for the flight, likewise an anxiety producing experience.
As can be seen, both the perceptual problem and the cost of modifying the aircraft, the cost of logistics, the cost of servicing and the cost of system calibration does not provide ready feasibility for aircraft-carried DIRCM type systems.
Aside from infrared laser-based countermeasure systems, other systems for protecting aircraft include LAMP-based DIRCM systems. However, the LAMP-based systems do not cover the required infrared band necessary for jamming modern missile seekers.
It is of course possible and not very expensive to eject flares as decoys to countermeasure shoulder-launched missiles. However, utilizing flares over a populated area is impractical because the flares can start fires. Thus flare type countermeasures are not acceptable in an urban environment.
Some have proposed to put up an IR chaff cloud of hot metal particles that radiate in the infrared region of the electromagnetic spectrum. However, every time an aircraft is to descend below 10,000 feet to land the idea of dumping hot metal out of the tail of the aircraft is unacceptable especially over populated areas.
Another potential solution is to illuminate a portion of the wing with a laser to create a false target on the wing. While analysis supports the fact that an aircraft can survive a missile hitting the wing, while the aircraft might survive, the airline industry could not advocate such a solution.
Another type of countermeasure device which has been proposed is providing a fuel-fired mantle which involves towing an IR radiator behind the aircraft. However, the cost and complexity of such a system is a deterrent for such an application; and one cannot conceive of landing a plane towing a radiator behind it.